Powershift transmissions have a plurality of gears, shafts and clutch packs which are engageable in a number of different patterns to achieve distinct input/output drive ratios known as transmission gear ratios or simply "gears". In such transmissions, any given gear is selected by providing a proper pattern of electrical signals to selected solenoid actuators on a bank of hydraulic valves. These electrically-controlled transmissions overcome the problems associated with mechanically linking operator-actuated controls to a large size multiple-gear transmission, which is the type of transmission required in many kinds of heavy-duty off-road vehicles.
Microprocessor-based electronic controllers for powershift transmissions have been used for a number of years by various companies. One example of such a design is found in U.S. Pat. No. 4,425,620 entitled "Electronic Control for Power Shift Transmission", which is hereby incorporated by reference. That patent discloses a microprocessor-based electronic control system which has a mode select lever and an upshift-downshift pulser lever by which the operator may indicate the desired vehicle direction and gear in which a powershift transmission is to be operated.
When an operator momentarily disengages the clutch of a powershift transmission under a load and thereafter re-engages the clutch, a conventional powershift transmission produces a jolt when the speed of the output shaft of the transmission is not matched with the ground speed. The electronic controller described in the foregoing patent substantially eliminates the jolt by automatically selecting a gear of the transmission which causes the transmission output to match closely the ground speed of the off-road vehicle. The patent discloses a particular use of a ground-speed matching algorithm designed to help eliminate such jolts. However, the disclosed control system does not provide an electronically controlled gradual engagement of the clutch of the powershift transmission during gear shifts, it simply operates the solenoids in an on/off manner. In conventional powershift transmissions, the rate of clutch engagement is dependent upon hydraulic and mechanical controls such as orifices and one or more accumulators. By themselves such mechanical controls are not very effective under all conditions for providing smooth or jolt-free gear shifts.
The use of proportional actuator devices, such as hydraulic valves operated by torque motors is known in the agricultural and construction equipment art to provide relatively soft engagements thus helping avoid jolts during gear shifts. Such proportional actuation devices are frequently operated by pulse width modulated (PWM) signals whose duty cycle is varied in proportion to the desired average or DC value desired to be produced by the actuator. An electronic control system using such PWM signals to operate a powershift transmission for jolt-free soft engagements is described in U.S. Pat. No. 4,855,913 entitled "Electronic Control System for Powershift Transmission", the disclosure of which is hereby incorporated by reference.
While aforementioned electronic controllers for powershift transmissions have proved useful, they are only controlling one-half of the over-all engine/transmission combination, namely the transmission and not the engine. When the engine and transmission are considered in combination, a different set of problems is presented, as shown by the discussion below.
In older diesel power plants used for heavy-duty off-road vehicles, the horsepower-to-engine speed curve peaked at a relatively narrow band of engine speeds. This is illustrated by torque curve 36 and horsepower curve 38 in FIG. 1, which show a power band 40 of curve 36 above horizontal line 42 where the engine is almost exclusively operated. This relatively narrow power band 40 occurs over an engine speed range from about 1860 RPM to about 2000 RPM, at which the vehicle operates most efficiently that is the range at which the greatest amount of usable pulling power can be extracted from the engine. This kind of torque-to-engine-speed characteristic represented by curve 36 effectively forced the driver who needed to operate such a vehicle near its maximum power rating to find just the right gear in which to run the transmission, in order to be able to provide sufficient torque from the engine to carry out the tasks at hand.
Because such older engines only ran efficiently in a rather narrow band of speeds, the problem of how to extract near-maximum power from a vehicle was solved by the development of powershift transmissions having more and more gears, such as 12, 16, 24 and even 36 gears. Providing such additional gears presented a second problem of adding significantly to the cost of the transmission. But the benefits of being able to extract the maximum power from the engine over a wide range of speeds, and being able to run at a particular desired speed and obtain almost full power from the engine, were perceived sufficiently worthwhile to justify such increased transmission costs.
Transmissions having a large number of gears proved very popular. In some applications, particularly in certain agricultural tractor operations, a driver often wants to run the vehicle at a relatively constant speed without having to adjust the throttle or gear for variations in the load being experienced by the power plant. Such applications may involve planting, applying fertilizers, or harvesting crops, or others where the uniformity of the results achieved is fairly dependent upon maintaining a relatively constant vehicle ground speed. In other applications, especially farming, the operator's attention is taken up by the tasks of steering, or watching the implements hooked up to the off-road vehicle. Further, in some applications the operator wants the vehicle to run at a constant speed in a single gear in order to minimize wear-and-tear on the power plant. In still other applications, any shifting of gears effectively can only be accomplished by stopping the operation in progress. For example, when deep plowing a field, a tractor cannot easily switch gears since the tractor would immediately stop moving due to the braking action created by the plow in the ground. For these and other reasons, many farmers have grown used to setting their tractors to run in a single gear and at a single speed setting for a given application. The foregoing practices presented the problem of excessive fuel consumption.
Recently, manufacturers of large diesel engines, such as Cummins Engine Company of Columbus, Indiana, have been able to create engines having a broad and relatively flat torque-to-engine-speed curve. These advances are made possible in part by advances in electronic controls for engines. Cummins has a subsidiary, Cummins Electronics in Columbus, Ind., which designs and manufactures electronic engine control systems for large diesel engines. In FIG. 1, torque curve 44 and horsepower curve 46 illustrate the kind of broad performance range obtainable with such newer engines. Note that torque curve 44 has a large, relatively flat or constant torque portion 48, which is above horizontal line 42 from about 1700 RPM to about 2100 RPM.
The advent of such engines for off-road vehicles presents new opportunities to overcome the problems mentioned above, by optimizing drive train performance and increasing the number of modes in which an off-road vehicle equipped with a powershift transmission may be operated.
It is the principal object of the present invention to provide increased flexibility in the operation of large, off-road vehicles by coordinating the operation of the powershift transmission and an electronically-controlled engine via an electronic transmission control system. An important related object of the present invention is to provide for multiple modes of off-road vehicle operation, including a maximum horsepower mode and a fuel economy mode, by using the transmission control system as a master controller.
Another object of the present invention is to employ the broad torque versus engine speed characteristics of advanced diesel engines to obtain nearly infinite speed selection and gear adjustment in large off-road vehicles, especially tractors and other agricultural equipment, with fewer gears than required in conventional engine/powershift transmission combinations.
Yet another object is to provide a pseudo-gear mode in an engine/transmission combination which makes it appear to the operator that the off-road vehicle has more gears than actually exist.